1. Field of the Invention
The present invention is generally related to predistortion systems and in particular to an automatic predistortion system for a processing circuit.
2. Description of the Related Art
Signals, when are applied to circuits (e.g., electrical, optical), are often distorted as a result of being processed by the circuits. The distortion comprises any undesired signals produced by these processing circuits which are added to or are somehow combined with the applied signals. A well known technique for substantially canceling distortion produced by a processing circuit is a feedforward control system which has at least two feedforward loops coupled to the processing circuit to substantially reduce distortion produced by the processing circuit. The main problem with this technique is that each particular processing circuit requires its own separate set of feedforward control loops and thus the complexity of the control system significantly increases for applications requiring a plurality of processing circuits. Furthermore, the feedforward systems often need associated circuitry for each of the feedforward loops which circuitry provides control signals and information to the feedforward loops for reducing the distortion produced by the processing circuit; this further increases the complexity of the control system. Also, the associated circuitry often makes use of pilot signals applied to the processing circuit and the information is derived from the processed pilot signal. The pilot signal often interferes with signals applied to the processing circuit and thus still more circuitry is needed to control the generation and proper application of the pilot signal.
What is therefore needed is a system which is of relatively low complexity and is able to substantially reduce distortion produced by one or a plurality of processing circuits.
The present invention provides a method and apparatus for substantially reducing distortion produced by at least one processing circuit. The present invention is configured as an automatic predistortion system which comprises a first circuit configured to have substantially the same physical characteristics as the at least one processing circuit. Distortion produced by this first circuit, which is therefore very similar to the distortion produced by the at least one processing circuit, is isolated by a first feedforward loop coupled to the first circuit. A second feedforward loop coupled to the first feedforward loop, the first circuit and the at least one processing circuit is used to modify the isolated distortion. A scanning circuit coupled to the first circuit, the feedforward loops and the at least one processing circuit is configured to obtain information that is used to modify the isolated distortion. The information is obtained from a side band of signals applied to the automatic predistortion system and appearing at the output of the predistortion system such that when the modified isolated distortion is applied to the at least one processing circuit, the modified isolated distortion combines destructively with the distortion produced by the at least one processing circuit thus substantially reducing the distortion produced by the at least one processing circuit.